1. Field of the Invention
The present invention relates to a mounting board and a method of producing the same. More particularly, the invention relates to a mounting hoard which ensures excellent connection reliability and on which small electronic components can be mounted.
Priority is claimed on Japanese Patent Application No. 2007-334593, filed Dec. 26, 2007, the content of which is incorporated herein by reference.
2. Description of the Related Art
Electronic components have been mounted on a circuit board or other substrate using solder or a conductive adhesive.
An exemplary connection terminal structure is shown in FIG. 12, which is disclosed in Patent Document 1. This connection terminal structure includes a wiring board 101 having a wiring pattern formed thereon. The wiring pattern includes first lands 102. Second lands 103 made from a metallic material with poor solderability are provided on the first lands 102. Third lands 104 made from precious metal material with good solderability are provided on the second lands 103 for soldering. The second lands 103 define a surface to exclude solder pieces 106 at an outer periphery of the third lands 104. Electrode sections 141 of an electronic component 140 and the wiring board 101 are electrically connected together via the solder pieces 106. In this connection terminal structure, the wiring board 101 is made from epoxy resin, ceramic or alumina, and the wiring pattern therefore is a copper circuit. Accordingly, the structure disclosed in Patent Document 1 addresses typical problems relating to soldering on copper wiring, including a problem of separation of lands for soldering (namely, the third land 104) and the solder pieces 106.
In the case in which some electronic components are mounted on a membrane circuit board, the membrane circuit board includes a polyethylene terephthalate (hereinafter referred to as PET) substrate on which a printed silver paste circuit is formed. In mounting the electronic components by soldering, low-temperature solder (with a soldering temperature of about not more than 165° C.) must be used that causes no PET embrittlement.
Accordingly, for the mounting on the membrane circuit board, a conductive adhesive which starts a curing reaction at a relatively low temperature (i.e., about not more than 165° C.) is used instead of solder. FIG. 13A is a cross-sectional view schematically showing an exemplary mounting board in which an electronic component is mounted on a membrane circuit board using a conductive adhesive. A conductive paste circuit 220 provided on a substrate 210 is electrically connected to electrode sections 241 of an electronic component 240 via conductive adhesive pieces 260(260a). Outer peripheral areas of the conductive adhesives 260a are sealed with sealing resin 261 adhering thereto.
In this case, adhesive strength between the electronic component 240 and the substrate 210 is determined by epoxy resin contained in the conductive adhesive pieces 260 or by the sealing resin 261. Unlike connection by soldering, no metal connection is established in the mounting board of FIG. 13A, a connection state is therefore unstable that can cause poor conductivity.
Recently, more and more tin-plated electrode sections such as the electrode sections 241 of the electronic component 240 are used along with gold-plated electrode sections. The conductive adhesive pieces 260 typically include silver powder and epoxy resin as principal components. The silver component in the conductive adhesive pieces 260 is incompatible with the tin component and they may cause galvanic corrosion. In addition, moisture ingression into an epoxy resin component at an interface of the conductive adhesive pieces 260 and the connecting sections of the electrode sections 241 may cause local cell corrosion (i.e., galvanic corrosion). As a result, tin may be dissolved or oxidized to cause poor connectivity. In order to prevent this phenomenon, a binder with reduced hygroscopicity should be employed to inhibit moisture ingression.
The conductive adhesive pieces 260a may be deformed and extended during mounting of the electronic component 240. In a subsequent heating process, the conductive adhesive pieces 260a may lose viscosity and then flow out of the applied place. As a result, two conductive adhesive pieces 260a are flown into space 250 to unite together as shown in FIG. 13B. To prevent occurrence of such a phenomenon, the distance between the two conductive adhesive pieces 260a should be increased to some extent. It is therefore difficult to mount small electronic components on the conventional mounting board.    [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2006-120677
The invention has been made in view of the aforementioned circumstances, and an object thereof is to provide a mounting board which ensures excellent connection reliability and on which small electronic components can be mounted.
Another object of the invention is to provide a method of producing a mounting board on which tin-plated small electronic components can be mounted with improved yields of products.